Conventionally-known, hydraulic working machines include those provided with a travelling state detection means for detecting their travelling state and a selector valve for changing over a pilot line, through which a pilot signal for lowering a boom (in a direction to cause contraction of a hydraulic cylinder for the boom) is transmitted, to a cut-off position or a communicating position on the basis of a signal from the travelling state detection means such that, when a combined operation of a travelling operation and a boom-lowering operation is performed by changing over the selector valve to the pilot-line-communicating position upon detection of a travelling state by the travelling state detection means, pressure oil is fed from a main pump to a rod chamber of the hydraulic cylinder for the boom to permit a jack-up of a body by the boom (see, for example, Patent Document 1-JP-A-06-002344, FIG. 1).
The conventionally-known, hydraulic working machines also include those of the type that upon lowering a boom, return oil from a bottom chamber of a boom cylinder is regenerated in a rod chamber of the boom cylinder to permit avoiding fluctuations in boom operating speed, which would otherwise take place as a result of changes or the like in an external force applied to the boom cylinder, while making it possible to reduce the horse power consumption of a main pump (see, for example, Patent Document 2-JP-A-05-302604, FIG. 1).